LONDON, England --A recent surge in suicide bombers from the breakaway Republic of Chechnya has caused security to be beefed up across Russia.

But the war once fought by men and now waged by women -- willing to kill themselves and others -- has caused shock and fear in the Russian capital.

Last week three female bombers linked to Chechen rebels were involved in two separate incidents in Moscow which killed 15 people.

The suicide bombers -- who have become known as "black widows" -- are dressed head-to-toe in black and wear the so-called "Martyr's belt" filled with explosives.

They were first seen in the Russian capital at last October's hostage-taking in a Moscow theater.

Kremlin officials say international terrorism is the reason for the increase in women bombers.

"The techniques, the financing, the outside control definitely comes from abroad," said Kremlin official, Alexander Mochevsky.

But many believe these women are being exploited by the terrorists.

"This is absolutely not characteristic of Chechens," said Aslanbek Aslakhanov, a member of the Russian parliament.

"Men never send their women to fight in wars. There is no religious aspect to this -- its psychological ... terrorists exploiting the misfortune of these women," Aslakhanov said.

Some believe Chechen women who have lost husbands and brothers in the war are desperate to seek revenge.

"There is a line of (young women) hoping to be chosen as candidates for being suicide bombers ... They say they want to force Russians to feel the same pain they have felt," journalist Anna Politkovskaya told CNN.

On Wednesday, a Chechen woman -- believed to be in her 20s -- tried to smuggle a bag containing a bomb into a busy restaurant in the heart of the capital.

A bomb disposal expert from Russia's Federal Security Service was killed when the bomb -- with an explosive force of 400 grams of TNT went off. (Full story)

Last Saturday, two female suicide bombers killed 14 people at a rock concert in Russia where more than 20,000 people were attending an annual music festival.

Both bombers tried to enter to the concert at Tushino air field, north of Moscow -- but were not allowed inside -- and detonated the explosives at entrances to the venue. (Full story)

It was the latest in a series of suicide attacks that have taken place in Russia and its breakaway Republic of Chechnya.

Last month, a woman killed at least 17 people after throwing herself under a bus carrying members of Russia's military near Chechnya and detonating explosives.